1. Field of the Invention
The present invention generally relates to an exhaust system. More particularly, but not exclusively, the present invention relates to an exhaust system for an internal combustion engine having a main exhaust passageway and a bypass exhaust passageway.
2. Background Information
It is known for exhaust systems of internal combustion engines to be provided with a catalytic converter arranged relatively downstream within the exhaust system for purifying the exhaust gases emitted by the engine, thereby to reduce the emission of environmental pollutants. The performance of the catalytic converter in purifying the exhaust gases depends on its being “activated”, which requires the temperature of the catalytic converter to be raised to a predetermined level, usually by heat transfer from the exhaust gases passing through it. On the other hand, excessive heating of the catalytic converter can cause deterioration to the catalyst.
It follows, therefore, that the positioning of the catalytic converter within the exhaust system may be an important factor in its performance. For example, if the catalytic converter is disposed too far upstream of the exhaust system, i.e. closer to the engine, durability may be reduced due to thermal deterioration of the catalyst. Conversely, the further downstream that the catalytic converter is located within the exhaust system, the longer it takes for the catalytic converter's temperature to be raised to a level sufficient for it to be activated after cold starting of the engine. As a result, the desired level of exhaust purification may not be realized during this time.
Japanese Patent Application Laid-Open No. 5-321644 discloses an exhaust system which is intended to address this problem. In this known exhaust system, a bypass exhaust path or passageway is provided in parallel with a portion of a main exhaust path or passageway upstream of a main catalytic converter disposed therein. A bypass catalytic converter is provided in the bypass exhaust passageway. In this arrangement, immediately after cold starting, exhaust gases are guided or diverted through the bypass exhaust passageway by a bypass control valve which is operable between a first position, in which the bypass exhaust passageway is open and the main exhaust passageway is substantially closed off, and a second position, in which the main exhaust passageway is open and the bypass exhaust passageway is substantially blocked off. Since the bypass catalytic converter is located relatively more upstream than the main catalytic converter in the exhaust system, and hence is activated more quickly following engine cold starting than the main catalytic converter, then exhaust purification can advantageously be started more rapidly. Once the internal combustion engine has warmed up and/or reached its optimal operating temperature, the bypass control valve shifts from the first position to the second position thereby to guide the exhaust gases to the main exhaust passageway and hence through the main catalytic converter.